Electric igniter and method of manufacturing same

ABSTRACT

The present invention relates to an electric igniter including: a heat-generating body which generates heat by being electrified, an accommodation chamber in which the heat-generating body is disposed and exposed at the bottom surface, an explosive material being accommodated in the accommodation chamber, having a first layer in the heat generating body side and a second layer which is an upper layer of the first layer, the first layer formed by loading a first explosive material in a dry state, and the second layer formed by loading a second explosive material in a wet state.

This nonprovisinoal application claims priority under 35 U. S. C. § 119(a) on Patent Application No. 2006-232212 filed in Japan on Aug. 29, 2006 and 35 U.S.C. § 119(e) on U.S. Provisional Application No. 60/827,326 filed on Sep. 28, 2006, which are incorporated by reference.

BACKGROUND OF INVENTION

1. Field of Invention

The present invention relates to an electric igniter for use in an airbag inflator or a gas generator for a seat belt pretensioner for an automobile.

2. Description of Related Art

U.S. Pat. No. 5,648,634 discloses a method of manufacturing an electric igniter for use in an airbag or a pretensioner for an automobile, wherein a slurry-like primer (or slurry-like primer and a flash charge) and a powdered output charge are sequentially supplied to a bridge wire (heat-generating body) and an explosive material having a two-layer structure or three-layer structure is loaded.

WO-A1 No. 01/031,282 discloses an electric initiator having a structure in which a state of intimate contact of a bridge wire and an ignition agent is improved by loading the ignition agent under pressure into a cavity (accommodation portion of an igniter) having the bridge wire (heat-generating body) disposed inside thereof and in that the primer is omitted, an electric initiator assembly using such electric initiator, and methods for manufacturing the electric initiator and the assembly.

SUMMARY OF INVENTION

The present invention relates to an electric igniter including:

-   -   a heat-generating body which generates heat by being         electrified,     -   an accommodation chamber in which the heat-generating body is         disposed and exposed at the bottom surface,     -   an explosive material being accommodated in the accommodation         chamber, having a first layer in the heat generating body side         and a second layer which is an upper layer of the first layer,     -   the first layer formed by loading a first explosive material in         a dry state, and the second layer formed by loading a second         explosive material in a wet state.

The present invention also relates to a method of manufacturing an electric igniter containing a heat-generating body that generates heat by being electrified, an accommodation chamber in which the heat-generating body is disposed and exposed at the bottom surface and an explosive material accommodated in the accommodation chamber, an opening portion being provided in the accommodation chamber, including a step of loading the explosive material from the opening portion of the accommodation chamber by initially loading a first explosive material in a dry state to form a first layer on the heat-generating body and then loading a second explosive material in a wet state to form a second layer on the first layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein:

FIG. 1 shows a vertical cross-sectional view of an electric igniter; and

FIG. 2 shows a vertical cross-sectional view of igniter main body used in the igniter of FIG. 1.

DETAILED DESCRIPTION OF INVENTION

With the method of manufacturing an electric igniter in U.S. No. 5,648,634, by using an ignition agent (primer, flash charge) having high ignition sensitivity with respect to electrostatic charges and heat generation in a state of a slurry, it is possible to reduce the risk of the ignition agent being ignited by electrostatic charges and to improve safety of the manufacturing process.

However, voids easily appear in the slurry-like ignition agent when the agent is dried, and where voids appear between the bridge wire and the ignition agent, ignition malfunction such as ignition delay in the ignition agent can be caused. Further, in order to eliminate void generation during drying, with the method of manufacturing an electric igniter disclosed in US-B2 No. 6,698,356, a slurry-like ignition agent is loaded into an accommodation portion therefor and dried, while the accommodation portion is rotated about the axis thereof, whereby the generation of voids is inhibited. Such a method is, however, complex because it involves a rotation operation.

With the method of manufacturing an electric igniter of WO-A1 No. 01/031282, a tight contact state of the bridge wire and ignition agent is improved by loading the ignition agent into the cavity. However, when an ignition agent in a dry state is loaded under pressure (in particular, when an indenter is repeatedly pushed into the cavity filled with the ignition agent), the ignition agent easily overflows the upper edge of the cavity, and such overflowing occurs easier as the amount of ignition agent loaded into the cavity is increased (the upper portion of the loaded ignition agent becomes closer to the upper edge of the cavity).

The ignition agent in a dry state can be ignited by dust explosion or static electricity, and in order to maintain the safety of manufacturing process, an operation of recovering the overflowed ignition agent should be frequency performed and each time the production equipment has to be stopped, thereby adversely affecting the productivity.

Further, when an increased amount of ignition agent is loaded at once, load applied to the cavity increases. As a result, structural materials having a higher strength or larger thickness should be used to increase resistance of the cavity to pressure. Therefore, material cost, weight, and dimensions of the igniter are increased.

Addition of safety measure specifications such as prevention of ignition caused by static electricity raises the cost of equipment that handles a dry ignition agent as compared with that of production equipment without safety measure specifications. When the loading operations are divided into a plurality of steps to adapt to the increase in the amount of ignition agent loaded in one igniter, the number of loading steps increases, and when the process is performed continuously, multistage loading specification or plurality of loading equipments is required, thereby also causing increase in production cost.

The present invention relates to an electric igniter that can be manufactured safety and with good productivity and also to a method of manufacturing such an electric igniter.

In the electric igniter in accordance with the present invention, because the loaded amount of the first explosive material in a dry state that forms the first layer can be decreased, the first explosive material is prevented from overflowing form the accommodation chamber when the first explosive material is loaded or pressed in the accommodation chamber, and thereby, operational safety is increased.

In the electric igniter in accordance with the present invention, because the second explosive material forming the second layer is in a wet state, no ignition occurs even when the second explosive material overflows from the accommodation chamber during loading, and operational safety is increased by comparison with that attained with the first explosive material in dry state.

It is preferable that the second explosive material comprises a fuel, an oxidizing agent and a binder.

With the electric igniter in accordance with the present invention, even when the second explosive material is dried after loading and voids appear in the second layer, because the first layer is pressed against the heat-generating body of the igniter, a state of intimate contact is maintained between the heat-generating body and the entire explosive materials and excellent ignition ability is maintained.

In the electric igniter in accordance with the present invention, the loaded amount of the first explosive material in a dry state can be decreased. Therefore, safety of the entire process is increased and the entire load applied to the accommodation chamber during loading operation is also decreased.

EMBODIMENT OF INVENTION

Electric Igniter

FIG. 1 is a vertical sectional view of the electric igniter in accordance with the present invention. FIG. 2 is a vertical sectional view of the igniter main body 10 used in the igniter of FIG. 1.

As shown in FIG. 1, in an electric igniter 1, the igniter main body 10 is fixed to a metallic igniter collar 21 via a resin 22. The reference numeral 24 stands for a space for inserting a connector.

As shown in FIG. 2, the igniter main body 10 has a metallic header portion 11 connected to a conductive pin 13 a and a glass insulating portion 12 connected to a conductive pin 13 b. A bridge wire 14 serving as a heat-generating body bridges between an end surface of the conductive pin 13 b and the metallic header portion 11. The bridge wire 14 is fixed by resistance welding.

A cylindrical charge holder 15 is disposed on the header portion 11. The charge holder 15 is fixed by ultrasonic welding after having a circumferential edge of one end surface thereof placed on the header portion 11. The other end side of the charge holder 15 is open, and the inside thereof serves as an accommodation chamber 16.

A first layer 17 including a first explosive material and a second layer 18 having a second explosive material are formed in the accommodation chamber 16. The first layer 17 is pressed against the bridge wire 14, and the first layer 17 and second layer 18 are in contact with one another.

The first layer 17 is obtained by loading the first explosive material in a dry state, and the second layer 18 is obtained by loading the second explosive material in a wet state. Preferably, the second explosive material is dried after loading. The first explosive material and second explosive material includes a fuel, an oxidizing agent, and optionally other additives, but the second explosive material preferably includes a binder as the other additive.

The first explosive material and second explosive material may have identical or different compositions. For example, by using a material with high ignition sensitivity as the first explosive material, the first layer 17 can mainly demonstrate an ignition function. And by using a material generating a high output as the second explosive material (ignition sensitivity may be lower than that of the first explosive material), the second layer 18 can mainly demonstrates an output function.

No specific limitation is placed on the thickness ratio of the first layer 17 and second layer 18, but when, as described above, the first layer 17 is imparted mainly with an ignition function and the second layer 18 is imparted mainly with an output function, it is preferred that the thickness of the second layer 18 be larger than that of the first layer 17, and the thickness of the second layer 18 can be 1 to 10 times that of the first layer 17.

Method of Manufacturing the Electric Igniter

In the method of manufacturing the electric igniter 1 shown in FIG. 1, a method of manufacturing an electric igniter by using the finished igniter main body 10 is well known (however, the igniter main body 10 shown in FIG. 2 is not well known) and is disclosed, for example, in JP-A No. 2004-293835.

A specific feature of the manufacturing method in accordance with the present invention is that it includes a process in which when an explosive material is loaded into the accommodation chamber 16 at the time of manufacturing the igniter main body 10, a first explosive material in a dry state is initially loaded to form the first layer 17 on the bridge wire 14, and then a second explosive material in a wet state is loaded on the first layer 17 to form the second layer 18. Other components are assembled by well-known methods.

First, a circumferential edge at one end of the cylindrical charge holder 15 is fixed to the header portion 11 after the header portion 11, insulating body 12, two conductive pins 13 a, 13 b, and bridge wire 14 have been integrally assembled.

Then the predetermined amount of the first explosive material in a dry state is loaded in the accommodation chamber 16 and the first layer 17 is formed. In this process, the first explosive material is pressed from the upper side of the accommodation chamber (for example, pressed under a pressure of 10 to 200 MPa) and the first layer 17 is tightly pressed against the bridge wire 14.

No specific limitation is placed on the loaded amount of the first explosive material, that is, a thickness of the first layer 17, but from the standpoint of preventing the first explosive material from overflowing from the accommodation chamber 16 during loading or pressing operation, it is preferred that the loaded amount be not more than ⅔, more preferably not more than ½, and even more preferably not more than ⅓ of the depth of the accommodation chamber 16 (distance between the bottom surface where the bridge wire 14 is present to the opening of the charge holder 15).

The first explosive material is preferably in a powder form and contains as little water as possible. Further, it is preferred that the first explosive material mainly demonstrates an ignition function and has high ignition sensitivity induced by heat generated by the bridge wire 14. The first explosive material includes a fuel and an oxidizing agent and can optionally contain other well-known additives.

The fuel can be selected from zirconium, iron, tin, manganese, cobalt, nickel, tungsten, titanium, magnesium, aluminum, niobium, and mixtures thereof. Among the above, zirconium is preferred.

The oxidizing agent can be selected from chlorates such as potassium chlorate, perchlorates such as potassium perchlorate, lithium perchlorate, and sodium perchlorate, nitrates such as potassium nitrate, and mixtures thereof. Among the above, potassium perchlorate is preferred.

Then, the second layer 18 is formed on the first layer 17 by loading the second explosive material in a wet state. A drying step is performed, if necessary, after the second explosive material has been loaded.

For example, a method disclosed in JP-A No. 2004-115001 (Paragraph No. 37 to 43, FIGS. 2(a) to (d) can be applied as a method of forming the second layer 18 after loading the second explosive material in the form of slurry.

The second explosive material is in the form of slurry obtained by mixing a solvent and a composition including a fuel, an oxidizing agent and preferably a binder and further including, if necessary, a well-known additive.

By introducing a binder into the second explosive material, it is possible to disperse the fuel and oxidizing agent uniformly when the slurry is obtained. Further, because viscosity can be set within an advantageous range, operability during loading the second explosive material into the accommodation chamber 16 can be increased. In addition, the loaded second explosive material can be brought into intimate contact with the inner wall surface of the accommodation chamber 16.

In addition to the fuel and oxidizing agent used for the first explosive material, the second explosive material can include synthetic explosives such as metal oxides, e.g. copper oxide, cyclotrimethylene trinitramine (RDX) and nitroguanidine and mixed explosives such as single-base explosives.

Examples of a suitable binder may include cellulose resins, urethane resins, and furan-type rubber compositions. Hydroxypropyl cellulose, nitrocellulose and urethane resin are preferred. The content of the binder in the second explosive material differs according to the type of fuel or oxidizing agent, but is preferably 0.5 to 15 mass %, more preferably 1 to 10 mass %.

Examples of other additive may include glass powders, glass fibers, ceramic fibers, steel wool, bentonite, kaolinite, and mixtures thereof.

A preferable solvent has no adverse effect on the first explosive material that forms the first layer 17. For example, it may be a solvent including water or an organic solvent such as alcohols such as isopropyl alcohol, methyl ethyl ketone, hexane or ethyl acetate.

Viscosity of the slurry-like second explosive material is 1 to 500 Pa·s, more preferably 5 to 300 Pa·s.

In the manufacturing method in accordance with the present invention, a process of forming an intermediate layer between the first layer 17 and second layer 18 can be provided.

In the process of forming an intermediate layer, a material for forming the intermediate layer is disposed on the first layer 17 to form the intermediate layer, and the second layer 18 is formed on the intermediate layer, in order to prevent the solvent contained in the second explosive material from adversely affecting the first layer 17.

As the material for forming the intermediate layer, a combustible material of the same shape as the exposed surface of the first layer 17, for example, paper, nonwoven fabric, resin sheet, resin film, or metal foil can be used. The second explosive material enclosed in the aforementioned material for forming the intermediate layer may be also disposed on the first layer 17.

Then, after the second layer 18 has been formed, an aluminum cup member 20 is disposed to cover the entire side surface of the charge holder 15 and head portion 11. The cup member 20 and header portion 11 are welded and integrated in the contact portion thereof to prevent moisture permeation.

EXAMPLES Compound Example 1 Compound Example of Slurry-Like Second Explosive Material

Fuel: zirconium 141 g

Oxidizing agent: potassium perchlorate 137.6 g

Binder: hydroxypropyl cellulose 2 g

Solvent: isopropyl alcohol 64.2 g

Example 1

The igniter main body 10 (the accommodation chamber 16 is empty) shown in FIG. 2 was prepared. The depth of the cylindrical accommodation chamber 16 was about 5 mm.

First, the first explosive material (fuel: zirconium 55 mass %, potassium perchlorate 43 mass %, binder 2 mass %) was loaded into the accommodation chamber 16. Then, an indenter was used to apply a pressure of 9 MPa from above and the first layer 17 (having the thickness of 1.5 mm) was formed. Because the amount of the loaded first explosive material was less than the depth of the accommodation chamber 16, the first explosive material did not overflow from the accommodation chamber 16 during loading and pressing.

Then, the second explosive material of Compound Example 1 was prepared and dropwise added with a metering pump onto the first layer 17. Upon completion of the delivering process, the second explosive material was dried to form the second layer 18 (having the thickness of 3 mm). The appearance of some voids was observed in the second layer 18 during drying, but because the first layer 17 was present and the contact between the entire explosive material and bridge wire 14 was maintained, absolutely no effect was produced on ignition ability.

The aluminum cup member 20 was then covered and contact portion with the header portion 11 was welded.

The invention thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the sprit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. 

1. An electric igniter comprising: a heat-generating body which generates heat by being electrified, an accommodation chamber in which the heat-generating body is disposed and exposed at the bottom surface, an explosive material being accommodated in the accommodation chamber, having a first layer in the heat generating body side and a second layer which is an upper layer of the first layer, the first layer formed by loading a first explosive material in a dry state, and the second layer formed by loading a second explosive material in a wet state.
 2. The electric igniter according to claim 1, wherein the second explosive material comprises a fuel, an oxidizing agent and a binder.
 3. A method of manufacturing an electric igniter comprising a heat-generating body that generates heat by being electrified, an accommodation chamber in which the heat-generating body is disposed and exposed at the bottom surface and an explosive material accommodated in the accommodation chamber, an opening portion being provided in the accommodation chamber, comprising a step of loading the explosive material from the opening portion of the accommodation chamber by initially loading a first explosive material in a dry state to form a first layer on the heat-generating body and then loading a second explosive material in a wet state to form a second layer on the first layer.
 4. The method for manufacturing an electric igniter according to claim 3, wherein the second explosive material comprises a fuel, an oxidizing agent and a binder. 